A relay is an entity that performs a function of assisting signal transmission and reception between a donor base station and a user equipment. The relay can be introduced for the purpose of expansion in service coverage, improvement in throughput at a cell edge.
According to the related art, the relay is designed and used by considering that it operates at a fixed location. However, in the case of a mobile relay, it should be designed in consideration of not only variable locations but also a high speed movement. For instance, there may be a case that a relay is located in a high-speed train. In particular, in case that the location of the relay varies depending on time, a cell formed by the relay may have mobility and the cell can be referred to as a mobile cell. Moreover, in order to implement such a mobile relay, a method for a user equipment to operate as a relay has been discussed.
A Proximity service (ProSe) means a method of supporting communication between physically adjacent devices. Particularly, the ProSe aims at supporting operations of discovering an application running between the adjacent devices and exchanging application-related data between the devices. For instance, the ProSe is considered to apply applications for SNS (social network service), business, game and the like.
Moreover, the ProSe can be also referred to as D2D (device-to-device) communication. The D2D communication means a communication scheme that enables a plurality of devices (e.g., user equipments (UEs)) to directly exchange user data (e.g., voice data, multimedia data, etc.) without a network in a manner of establishing a direct link between the devices. ProSe communication can include UE-to-UE communication, peer-to-peer communication and the like. And, the ProSe communication can be applied to M2M (machine-to-machine) communication, MTC (machine type communication) and the like. Thus, the ProSe is considered as one method capable of solving a burden of a base station caused by rapidly increasing data traffic. Furthermore, if the ProSe is introduced, benefits such as reduction in a procedure of the base station, reduction in power consumption of devices participating in the ProSe, increase in a data transfer rate, increase in capacity of a network, load distribution, expansion in cell coverage, and the like may be expected.
A ProSe technology includes a UE-relay technology and the UE-relay technology means a technology for supporting a UE capable of operating as a relay (hereinafter referred to as UE-relay (UE-R)).